This heating is typically provided by a device comprising:
a magnetic circuit including an airgap,
transport means for moving the product to be heated through the air gap,
a coil surrounding said magnetic circuit in the vicinity of the air gap,
a capacitive system typically comprising a battery of capacitors and connected to the winding to constitute a circuit which resonates at an operating frequency which is generally between 100 and 1 000 Hz and typically around 250 Hz, and
an electrical generator feeding current into the resonant circuit at the operating frequency.
The presence of the capacitive system enables a current to be passed through the coil that is much higher than the current provided by the electrical generator. The latter then supplies only an active power that is actually consumed by the device, a "reactive" power of perhaps ten times this amount being provided by the capacitive system.
The product to be heated is often travelling at high speed and may feature irregularities which make it necessary to provide a wide gap. Also, the product temperature is often such that a thermally insulative layer must be provided to either side of the gap to protect the coil and the nearby electrotechnical equipment. As a result the airgap of the magnetic circuit must be large, which results in a high leakage of magnetic flux in the region of the coil. Part of this leakage flux is of no benefit for heating the product and induces current into the coil conductors which causes significant unwanted heating of said conductors.
To reduce this unwanted heating and to increase the energy efficiency of the device, in other words the ratio of the heating power developed by the current induced in the product to be heated to the active power supplied by the electrical generator, it is known:
to make the heating coil as compact as possible,
to use for the electrical conductors of the coil a form that is sufficiently subdivided, given their electrical resistivity and the operating frequency, in other words giving them sufficiently small transverse dimensions, to reduce the generation of induced current in the metal mass of each conductor, multiple conductors being then grouped in parallel and insulated from each other except at their two ends where they are joined to two terminals common to all the conductors of a group,
to transpose the conductors within the same group to reduce the induced current that may flow in a closed loop comprising two conductors and the two terminals of the group, and
to cool the coil strongly using a cooling circuit to enable a high usable heating power to be applied by means of a compact coil.
For this reason one known heating coil comprises certain features which are, as to their function as explained hereinafter, common to this coil and to a coil in accordance with the invention, these common features comprising:
a ferromagnetic core,
two electrical terminals adapted to receive an alternating current,
a group of electrical conductors connected in parallel between the two electrical terminals, the group being in the form of a winding around said feromagnetic core, the conductors being transposed within the group so as to equalize approximately the various alternating ma fluxes enclosed by the respective conductors of the group, the transposition being achieved by means of transposition deformations of said conductors in transposition areas of said conductors,
a cooling pipe around said core in thermal contact with said conductors, and
hydraulic terminals for circulating a cooling fluid in said cooling pipe.
U.S. Pat. No. 4,176,237 describes an induction furnace for liquid metals. It is provided with an inductive heating coil comprising conductors connected in parallel between two electrical terminals of the coil, each of the conductors comprising a cooling tube, the current carried by the conductor being divided between strands in thermal contact with the walls of the tube, the length of the conductor including strongly deformed areas in is subject to particularly marked deformations which transpose conductors and strands to reduce the formation of unwanted current loops.
Known coils of this kind leave much to be desired with regard to their compactness, their cost and the energy efficiency of the heating device of which they form part.
One object of the present invention is to enable the simple manufacture of a compact heating coil which reduces the energy losses of an inductive heating device.